Matriptase is a serine protease that is expressed in the epithelial components of essentially all epithelial tissues including the mammary gland. We have shown that in these tissues, Hepatocyte Growth factor Activator Inhibitor-1 (HAI-1), a kunitz-type protease inhibitor, is required for the normal regulation of matriptase expression and activation, and is responsible for inhibiting the enzyme after it becomes activated. We and others have shown that overexpression of matriptase is a marker of poor prognosis in a variety of human cancers, including breast cancer. When overexpressed in the skin of transgenic mice, matriptase has been shown to act as an oncogene - an effect blocked by HAI-1 overexpression. Several substrates activated by matriptase are believed to be involved in carcinogenesis and/or tumor progression, such as Hepatocyte Growth Factor, or Urokinase Plasminogen Activator, however, much remains to be discovered about the role of misregulated matriptase activity in cancer, and the underlying biochemical mechanisms involved. To explore the role of the matriptase/HAI-1 system in the mammary gland we have developed two transgenic mouse models in which we can induce the overexpression of HAI-1 or matriptase in mammary epithelial tissue. Using these models we have shown that altering the matriptase/HAI-1 balance can have a profound effect on mammary gland biology, function, and carcinogenesis. These findings are consistent with our previously published studies analyzing human tumor samples which demonstrated that overexpression of matriptase is associated with poor prognosis in node-negative breast cancer. This proposal will bridge the gap between these two bodies of work by developing an integrated understanding of the role of the matriptase/HAI-1 system in mammary gland biology, carcinogenesis and tumor progression, the biochemical mechanisms involved in these processes, and the clinical implications of their disruption in breast cancer. In Aim 1 we will use our transgenic systems to study the regulation of normal mammary gland biology, mammary carcinogenesis, and metastatic tumor behavior, by the matriptase/HAI-1 system, and will study the underlying mechanisms involved. In Aim 2 we will examine the role of hypoxia and aberrant glycosylation in the aberrant regulation of matriptase activity. In aim 3 we evaluate the clinical importance of our data by integrating the findings of the previous two aims with an analysis of primary human breast tumor samples. We will evaluate the utility of matriptase and HAI-1, alone or in combination with markers of hypoxia or altered glycosylation, as prognostic markers of clinical outcome in breast cancer. We believe that these studies will highlight the importance of the matriptase/HAI-1 system in breast cancer and will provide valuable insights as to how the pathway might be targeted therapeutically.